When a sticky piston with a block in front of it is powered by a one tick pulse it creates a second piston head. This happens for example when you power the sticky piston with an observer.

This does not happen ff there is no block in front of the piston or if it is a normal piston.

It does not matter which direction the piston is facing.

How to reproduce:
-Place a piston with a block in front of it.
-Power the piston with a observer